Discovery of a specific oligomannose cluster as the epitope of a broadly HIV-1 neutralizing antibody (2G12) has stimulated substantial interest in carbohydrate-based HIV-1 vaccines. Recently, we identified a significant population of Man9-cluster-binding B cells (BMan9+) in the naive B cell repertoires. In human peripheral blood, this population constitutes approximately 12% CD19+CD45+ B cells. Similarly, BMan9+ cells represent 10-12% of B-1a (CD19+CD5+) B cells in the mouse peritoneal cavity (PerC). Using carbohydrate microarrays to screen anti-glycan antibodies in human blood circulation, we revealed that significant amounts of anti-Man9-cluster antibodies (IgM and IgG isotypes) are present in both HIV-1 infected and non-infected human subjects. However, the levels of IgG antibodies to Man9-cluster antigen are markedly increased in the HIV-1 infected subjects. These findings suggest that there is an evolutionally conserved BMan9+ cell population present in the pre-existing B cell repertoires that is readily reactive with HIV-1 carbohydrates. Since thi population of antigen-binding B cells is extraordinarily larger than the generally observed antigen-specific B cell populations, we propose that these B cells express not only conventional Ig-B cell receptors but also non-Ig-mannose-binding receptor(s), such as DC-SIGN. Upon binding to either soluble gp120 glycoprotein or the native HIV virions, these cells enter into a unique T-independent pathway of B cell activation, i.e., the C-type-lectin-engaged T-independent type 1 response (TI-1c). In this study, we plan to make use of well-established IgH-allotypic mouse models to further characterize the TI-1c pathway of B cell activation and antibody responses to oligomannosyl antigens of HIV-1. Specifically, we will a) characterize the B cell lineage distribution and origin of the BMan9+ cells (Aim 1); b) examine contributions of B-1 and B-2 lineages to production of anti-oligomannose antibodies in the natural antibody repertoires (Aim 2); c) Examine the acquired antibody responses to rationally designed TI-1c oligomannosyl antigens of HIV-1 (Aim 3); and d) Determine the fine specificity and potential HIV-1 neutralization activity of vaccine-elicited anti-Man9-cluster antibodies (Aim 4). In summary, we propose and design experiments to characterize the TI-1c pathway of B cell activation and antibody responses to oligomannosyl antigens of HIV-1. Information obtained by this study is likely instructive to development of novel vaccination strategies against HIV-1 and other pathogens that express oligomannosyl sugar moieties.